Usually, a liquid crystal display device includes a liquid crystal cell that a liquid crystal component oriented in a predetermined direction is sandwiched between two supporting substrates such as glass, and an optical film such as a polarizing film, a phase difference film or a brightness improving film. An adhesive composition is frequently used at the time of lamination of the optical films and adhesion of the optical film to the liquid crystal cell.
The liquid crystal display device is used as a display device of a personal computer, a television set, a car navigation system and the like in a wide range. It sometimes happens that these devices are used in such a severe environment as a high temperature and high humidity. Consequently, it is required for an adhesive composition to have such excellent durability that separation and air bubbles do not occur even in a long-term use. In addition, in such a severe environment as a high temperature and high humidity, the optical film is subjected to increase in change of dimension such as shrinkage or expansion. The conventional adhesive composition layer cannot relax stress generated due to the change of dimension so that residual stress of the optical film becomes inhomogeneous. As a result, so-called “white spots” meaning a phenomenon that the peripheral part of liquid crystal display device is whitened due to the fact that a light is leaked from the part, have been problematic.
For improving the above-mentioned problem, an adhesive composition has been proposed, that has a stress relaxation property by that a low molecular weight polymer is added to the adhesive composition. For example, in Patent Literature 1 (JP-A-1998(Heisei-10)-279907), an adhesive composition for a polarizing plate that includes a high molecular weight acrylic copolymer, a low molecular weight acrylic copolymer having weight-average molecular weight of not more than 30,000 and a polyfunctional compound has been disclosed. However, the adhesive composition described in Patent Literature 1 contains a large amount of the low molecular weight acrylic copolymer having weight-average molecular weight of not more than 30,000 so that it is difficult to prevent the occurrence of bubbles and the separation.
As methods for inhibiting the white spots, there are a method of preventing the occurrence of the white spots by following the change of dimension of the polarizing film so as to relax concentration of stress as the adhesive composition described in Patent Literature 1, and to the contrary, a method of preventing the occurrence of the white spots by using an adhesive composition having high cohesion force so as to keep a region in which stress occurs at a minimum.
In Patent Literature 2 (JP-A-2007-112839), for improving adhesion properties of an adhesive layer, an adhesive composition has been disclosed, that includes 0.02 to 2 parts by weight of a peroxide, 0.02 to 2 parts by weight of an isocyanate cross-linking agent, and 1 to 40 parts by weight of a tackifier resin having a softening point of not less than 80 degrees C. relative to 100 parts by weight of an acrylic copolymer. However, the Patent Literature 2 fails to describe about the white spots, so that the ability to prevent the occurrence of the white spots is not satisfied only by having the above-mentioned configuration.